1. Field of the Invention
The present invention relates to insulating ceramic compacts for use in multilayer circuit substrates, and more particularly, relates to a high-frequency insulating ceramic compact which is suitably used for a hybrid multilayer circuit substrate for mounting semiconductor elements or various electronic elements thereon and which can be simultaneously fired with a conductive material such as copper or silver, relates to a ceramic multilayer substrate using the insulating ceramic compact and relates to a ceramic electronic device.
2. Description of the Related Art
In recent years, trends toward high speed and high frequency processing of electronic devices have been progressing rapidly. In addition, electronic elements mounted on electronic devices are required to satisfy a higher processing speed and a higher integration density and furthermore, are also required to satisfy a higher mounting density. In response to the requirements described above, multilayer circuit substrates have been used as substrates for mounting semiconductor elements and various electronic elements thereon. In multilayer circuit substrates, conductor circuits or functional electronic elements are embedded, and hence, miniaturization of electronic devices can be performed.
As a material for forming the multilayer circuit substrate described above, alumina has heretofore been used in many cases.
The firing temperature for alumina is relatively high, such as 1,500 to 1,600xc2x0 C. Accordingly, a high melting point metal, such as molybdenum (Mo), molybdenum-manganese (Moxe2x80x94Mn), tungsten (W) or the like, must be generally used as a material for a conductive circuit embedded in the multilayer circuit substrate composed of alumina. However, these high melting point metals have high electrical resistance.
Accordingly, it has been strongly desired that an inexpensive metal, such as copper, having a lower resistance than that of the high melting point metals be used as a conductive material. In order to use copper as the conductive material, usage of a glass ceramic or a crystallized glass, which can be fired at 1,000xc2x0 C. or less, is proposed (for example, Japanese Unexamined Patent Application Publication No. 5-238774).
In addition and in consideration of connection with a semiconductor element such as a silicon (Si) chip, usage of a ceramic having a coefficient of thermal expansion approximately equivalent to that of Si is proposed as a material for a multilayer circuit substrate (Japanese Unexamined Patent Application Publication No. 8-34668).
However, the known substrate materials described above, which can be fired at a low temperature, have problems in that the mechanical strength is low, the Q value is low and the types of the precipitated crystalline phases and the ratio thereof are easily influenced by the firing process.
In addition, the substrate materials disclosed in Japanese Unexamined Patent Application Publications Nos. 5-238774 and 8-34668 have a problem in that co-firing with a high dielectric material having a high coefficient of thermal expansion is difficult to perform.
Accordingly, an object of the present invention is to provide an insulating ceramic compact which can solve the problems of the conventional techniques described above, can be fired at a low temperature, can be simultaneously fired with a conductive material having a relatively low melting point, such as silver or copper, has a low relative dielectric constant and superior high-frequency characteristics, and in addition, has a high coefficient of thermal expansion.
Another object of the present invention is to provide a ceramic multilayer substrate which is formed of the insulating ceramic compact described above, can be fired at a low temperature, has a low relative dielectric constant and superior high-frequency characteristics and can be obtained by co-sintering with a high dielectric material having a high coefficient of thermal expansion, and is to provide a ceramic electronic device and a laminated ceramic electronic device which use the ceramic multilayer substrate described above.
Through intensive research by the inventors of the present invention in order to solve the problems described above, it was discovered that, in an insulating ceramic compact formed of a fired mixture of an MgAl2O4-based ceramic and a borosilicate glass, when an MgAl2O4 crystal phase and at least one of an Mg3B2O6 crystal phase and an Mg2B2O5 crystal phase are precipitated as primary crystal phases, or when an MgAl2O4 crystal phase, an Mg2SiO4 crystal phase and at least one of an Mg3B2O6 crystal phase and an Mg2B2O5 crystal phase are precipitated as primary crystal phases, an insulating ceramic compact can be obtained having a low relative dielectric constant, superior high frequency characteristics and a high coefficient of thermal expansion, whereby the present invention was made.
In accordance with one aspect of the present invention, an insulating ceramic compact is provided comprising a fired mixture of an MgAl2O4-based ceramic and a borosilicate glass, in which an MgAl2O4 crystal phase and at least one of an Mg3B2O6 crystal phase and an Mg2B2O5 crystal phase are precipitated as primary crystal phases.
In accordance with another aspect of the present invention, an insulating ceramic compact is provided comprising a fired mixture of an MgAl2O4-based ceramic and a borosilicate glass, in which an MgAl2O4 crystal phase, an Mg2SiO4 crystal phase, and at least one of an Mg3B2O6 crystal phase and an Mg2B2O5 crystal phase are precipitated as primary crystal phases.
In the present invention, the borosilicate glass preferably comprises boron oxide, silicon oxide and magnesium oxide. When an MgAl2O4 ceramic and a glass composition containing at least boron oxide (B2O3), silicon oxide (SiO2) and magnesium oxide (MgO) are combined together, the MgAl2O4 crystal phase and at least one of the Mg3B2O6 crystal phase and the Mg2B2O5 crystal phase can be precipitated as the primary crystal phases, or the MgAl2O4 crystal phase and at least one of the Mg2SiO4 crystal phase, the Mg3B2O6 crystal phase, and the Mg2B2O5 crystal phase can be precipitated as the primary crystal phases, whereby, in both cases, an insulating ceramic compact can be obtained having superior high frequency characteristics and a high coefficient of thermal expansion.
The borosilicate glass preferably comprises about 8 to 60 wt % of boron oxide calculated as B2O3, about 10 to 50 wt % of silicon oxide as SiO2 and about 10 to 55 wt % of magnesium oxide as MgO. In addition, the borosilicate glass more preferably comprises about 20 to 40 wt % of boron oxide.
In the borosilicate glass, the boron oxide preferably occupies about 8 to 60 wt % in the form of B2O3. The boron oxide serves primarily as a fusing agent. When the content of boron oxide is less than about 8 wt % in the form of B2O3, the melting temperature may be excessively increased in some cases and when the content is more than about 60 wt %, the humidity resistance may be degraded in some cases.
The silicon oxide preferably occupies about 10 to 50 wt % in the form of SiO2. In addition, the silicon oxide more preferably occupies about 13 to 38 wt %. When the content thereof is less than about 10 wt %, the chemical stability of the borosilicate glass tends to be decreased and when the content is more than about 50 wt %, the melting temperature of the glass may be increased in some cases.
The magnesium oxide preferably occupies about 10 to 55 wt % in the form of MgO. In addition, the magnesium oxide more preferably occupies about 35 to 53 wt %. MgO decreases a melting temperature when a glass is formed and is a constituent component of a crystal in the crystallized glass. In particular, an MgOxe2x80x94B2O3 compound shows a Qf value (product of the Q value and the frequency f) of tens of thousands GHz and is primarily responsible for realizing superior high frequency characteristics. When the content of MgO is less than about 10 wt %, the Q value may be decreased in some cases and when the content thereof is more than about 55 wt %, the precipitated amount of the crystal becomes excessive, and hence, the substrate strength may be decreased in some cases.
By adjusting the ratio of the magnesium oxide and the boron oxide, contained in the borosilicate glass, a Mg3B2O6 crystal phase or a Mg2B2O5 crystal phase can be selectively precipitated. That is, when the magnesium oxide exceeds the ratio MgO: B2O3=3:1 on a molar basis, the Mg3B2O6 crystal phase can be precipitated. On the other hand, when the boron oxide is exceeds the ratio MgO: B2O3=3:1, the Mg2B2O5 crystal phase can be selectively precipitated. When the ratio is approximately MgO: B2O3=3:1, the Mg3B2O6 crystal phase and the Mg2B2O5 crystal phase are both present.
The borosilicate glass described above preferably further comprises about 20 wt % or less of an alkali metal oxide. The alkali metal oxide serves to decrease the melting temperature during glass formation; however, when the content thereof is more than about 20 wt %, the Q value tends to decrease. As the alkali metal oxide described above, there may be mentioned Na2O, K2O, Li2O and the like. In addition, the sintering temperature can also be decreased. When the amount of the alkali metal oxide in the borosilicate glass is adjusted, the coefficient of thermal expansion can also be adjusted.
The borosilicate glass preferably further comprises about 30 wt % or less of zinc oxide calculated as ZnO. Zinc oxide serves to decrease a firing temperature. However, when the content of zinc oxide is more than about 30 wt %, the chemical stability of the glass may be decreased in some cases.
The borosilicate glass preferably comprises about 10 wt % or less of copper oxide in the form of CuO. Copper oxide serves to decrease a firing temperature; however, when the content thereof is more than about 10 wt %, the Q value may be degraded in some cases.
The borosilicate glass preferably further comprises about 20 wt % or less of aluminum oxide in the form of Al2O3. Aluminum oxide can improve chemical stability. However, when the content of aluminum oxide is more than about 20 wt %, a dense sintered body may not be obtained in some cases.
The ratio of the MgAl2O4-based ceramic to the borosilicate glass is preferably in the range of from about 20:80 to 80:20 on a weight basis. When the content ratio of the ceramic described above is less than about 20 wt %, the Q value tends to decrease and when the content ratio is more than about 80 wt %, by firing at a temperature of from 900 to 1,000xc2x0 C., the obtained insulating ceramic compact may not be sufficiently densified in some cases.
In said one aspect of the present invention, when the total crystal phases are assumed to be 100 wt % in the sintered body described above, about 5 to 80 wt % of the MgAl2O4 crystal phase and about 5 to 70 wt % of the Mg3B2O6 crystal phase and/or the Mg2B2O5 crystal phase are preferably precipitated, respectively. In the ranges described above, high reliability, superior sintering characteristics, a sufficient mechanical strength and a high Q value can be obtained. When the ratio of the MgAl2O4 crystal phase is less than about 5 wt %, the strength of the insulating ceramic compact may be decreased in some cases and when the ratio thereof is more than about 80 wt %, densification may not be performed by sintering at 1,000xc2x0 C. or less in some cases.
When the content of the MgAl2O4 crystal phase is less than about 5 wt %, the filler component is decreased and the amount of an expensive glass is increased, whereby the cost may be increased in some cases. When the content is more than about 80 wt %, densification may be difficult to perform at 1,000xc2x0 C. or less. In addition, when the content of the Mg3B2O6 crystal phase and/or the Mg2B2O5 crystal phase is less than about 5 wt %, since the reaction between the magnesium oxide (MgO) and the boron oxide (B2O3) does not sufficiently proceed, the sintering characteristics and the reliability may be decreased, and the Q value may also be decreased in some cases. In order to precipitate about 70 wt % or more of the Mg3B2O6 crystal phase and/or the Mg2B2O5 crystal phase, the amount of an expensive glass must be increased, and as a result, the cost is increased.
In said another aspect of the present invention, when the total crystal phases are assumed to be 100 wt % in the sintered body described above, it is preferable that about 5 to 80 wt % of the MgAl2O4 crystal phase be precipitated, and that the Mg2SiO4 crystal phase and at least one of the Mg3B2O6 crystal phase and the Mg2B2O5 crystal phase be precipitated so that the total precipitated amount thereof is about 5 to 70 wt %. In the ranges described above, superior sintering characteristics, sufficient mechanical strength, superior high frequency characteristics and a high coefficient of thermal expansion can be obtained. When the content of the MgAl2O4 crystal phase is less than about 5 wt %, the mechanical strength may be decreased in some cases and when the content is more than about 80 wt %, densification may not be performed at 1,000xc2x0 C. or less in some cases. When the total precipitated amount of the Mg2SiO4 crystal phase, the Mg3B2O6 crystal phase and the Mg2B2O5 crystal phase is less than about 5 wt %, since the reaction between the magnesium oxide (MgO) and the boron oxide (B2O3) does not sufficiently proceed, the sintering characteristics and the reliability may be decreased and the Q value may also be decreased in some cases. When the total precipitated amount is more than about 70 wt %, the amount of an expensive glass must be increased, and as a result, the cost is increased.
As the glass described above, a mixture obtained by calcining a glass composition at 700 to 1,000xc2x0 C. may also be used.
According to the present invention, since the MgAl2O4 ceramic and the predetermined borosilicate glass described above are used, an insulating ceramic compact can be obtained which can be formed by co-sintering with a low melting point metal material such as copper or silver and which has a sufficient mechanical strength, superior high frequency characteristics and a high coefficient of thermal expansion.
In addition, the obtained insulating ceramic compact preferably has a Q value of 700 or more at a measurement frequency of 15 GHz. When the Q value is 700 or more at 15 GHz, the insulating ceramic compact is preferably used for a circuit substrate used in a high frequency region, for example, in a frequency region of 1 GHz or more.
A ceramic multilayer substrate according to the present invention comprises a ceramic board having insulating ceramic layers composed of an insulating ceramic compact of the present invention and a plurality of internal electrodes formed in the insulating ceramic layers of the ceramic board.
In the ceramic multilayer substrate of the present invention, on at least one surface of each of the insulating ceramic layers described above, a second ceramic layer having a dielectric constant higher than that of the insulating ceramic layers is preferably provide.
In the ceramic multilayer substrate of the present invention, the plurality of internal electrodes is preferably laminated to each other with at least a part of the insulating ceramic layers provided therebetween so as to form a laminated capacitor.
In the ceramic multilayer substrate of the present invention, the plurality of internal electrodes may comprise capacitor internal electrodes which are laminated to each other with at least a part of the insulating ceramic layers provided therebetween so as to form a laminated capacitor, and coil conductors which are connected to each other so as to form a laminated inductor. The capacitor is preferably provided on the second ceramic layer (for miniaturization and increased capacitance).
A ceramic electronic device of the present invention comprises the ceramic multilayer substrate of the present invention and at least one electronic element which is mounted on the ceramic multilayer substrate and which forms a circuit together with the plurality of internal electrodes.
The ceramic electronic device of the present invention preferably further comprises a cap which is fixed to the ceramic multilayer substrate so as to enclose the electronic-element. As the cap, a conductive cap is preferably used.
The ceramic electronic device of the present invention preferably further comprises a plurality of external electrodes formed only on the bottom surface of the ceramic multilayer substrate and a plurality of conductors provided in throughholes, which is electrically connected to the external electrodes and which is electrically connected to the internal electrodes or to the electronic element.
A laminated ceramic electronic device of the present invention comprises a sintered ceramic body composed of the insulating ceramic compact of the present invention, a plurality of internal electrodes disposed in the laminated ceramic body, and a plurality of external electrodes which are formed on outside surfaces of the sintered ceramic body and which are electrically connected to some of the internal electrodes.
In the laminated ceramic electronic device of the present invention, the plurality of internal electrodes is disposed so as to be laminated to each other with ceramic layers provided therebetween, and hence, a capacitor unit is formed.
In addition to the internal electrodes forming the capacitor unit described above, the plurality of internal electrode preferably further comprises a plurality of coil conductors connected to each other so as to form a laminated inductor unit.